Problem: The scale on a map is 6cm : 4km. If two cities are 18cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 6cm on the map corresponds to an actual distance of 4km. A distance of 18cm on the map is the same as 3 $\cdot$ 6cm. The actual distance between the two cities is 3 $\cdot$ 4km, or 12km.